Fix bug when extending minimizer graph using w < k

[lcoombe]

• We mask the assembly prior to lower w extension using the synteny blocks
• Need to filter out small blocks to ensure that the bedtools slop has the expected behaviour
• For low 'w' extensions, it was possible for the 'slop' to attempt to extend longer than 2*w, since k > w in these cases
  • This tweak should fix that edge case
• Also updating CI for btllib 1.6.2, fixing macOS (requires clang-15+, and compilers only has clang-14 right now)